Tissue welding devices typically comprise a pair of tweezers, jaws or forceps that grasp onto and hold tissue therebetween. Tissue welding devices may operate with a heating element in contact with the tissue, or with an ultrasonic heater that employs frictional heating of the tissue, or with a bipolar electrode heating system that passes current through the tissue such that the tissue is heated by virtue of its own resistance. Tissue welding devices are used to heat the tissue to temperatures such that the tissue is either “cut” or “sealed”, as follows.
When tissue is heated in excess of 100 degrees Celsius, the tissue disposed between the tweezers, jaws or forceps will be broken down and is thus, “cut”. However, when the tissue is heated to temperatures between 50 to 90 degrees Celsius, the tissue will instead simply “seal” or “weld” to adjacent tissue.
An example of a tissue welding device is found in Published PCT patent applications WO 98/38935 and WO 01/12090, and is also found in the TLS™ Thermal Ligating Shears and the Cautery Forceps devices sold by Starion Instruments Corporation of Saratoga, Calif. An advantage of the Starion devices are that they can be used to simultaneously cut and seal the ends of a blood vessel.
In the Starion tissue welding devices, a resistance wire heating element is a disposed on the surface of one of two opposing working surfaces of a pair of tweezers or forceps. The blood vessel to be sealed or cut is held between the opposing working surfaces of the device, and the resistance wire is heated. The portion of the blood vessel that is immediately adjacent to the resistance wire will be heated to temperatures in excess of 100 degrees Celsius (thereby “cutting” through the blood vessel at this location). When heated to this temperature, the tissue protein structure breaks down. On either side of this “cut zone”, the tissue will only be heated to temperatures between 50 to 90 degrees Celsius. When heated to this lower temperature, the tissue proteins become denatured and thus bond together. Therefore, when accompanied by mechanical pressure caused by gently squeezing the two opposing working surfaces together, the tissue will “seal” together, thus forming a “seal zone” on either side of the central “cut zone”. In these two seal zones, the ends of the blood vessel will be sealed shut.
When heating tissue with a resistance wire positioned in direct contact with the tissue, the temperature that the tissue is actually heated to is dependent upon the watt density of the heater. Moreover, high watt densities are required in order to achieve high local temperatures, especially into the “cut” temperature range (in excess of 100 degrees Celsius). Because of the limitations of small, lightweight DC power supplies, a small diameter resistance wire is required to achieve sufficiently high resistance and resulting watt densities, especially for tissue cutting temperatures.
A disadvantage of using a small diameter resistance wire for tissue sealing and cutting is that the contact area between the wire and the surrounding tissue is small. Thus, heat from the wire is only applied to a small area of the tissue. It would instead be desirable to increase the area of tissue to which heating is directly applied. This would be especially beneficial, for example, when sealing the end of a blood vessel since the creation of a larger “seal zone” at the end of the blood vessel would help ensure the blood vessel remains sealed.
Unfortunately, simply increasing the contact area between the resistance wire and the surrounding tissue by increasing the diameter of the resistance wire would result in decreasing the resistance and the watt density of the wire, thus significantly limiting the wire's tissue heating ability. To counteract this decreased watt density problem, it would therefore be necessary to increase the power applied to the wire. Unfortunately, such increased power levels tend to exceed the limits of existing small, lightweight DC power supplies. What is instead desired is a system in which the surface area (where the heater contacts surrounding tissue) is increased, but without changing the resistance or compromising watt density while working within the limitations of existing small power supplies.
In other non-medical resistance heater systems, the heating element comprises a resistance wire heater that is surrounded by a high resistance, electrically insulating, high thermal conductivity material that is in turn surrounded by a protective outer metal sleeve. In such systems, current is passed through the central resistance wire which is thus heated. The heat is then conducted outwardly through the high thermal conductivity material and the outer metal sleeve. In such systems, current is not passed through the outer metal sleeve. A disadvantage of such systems is that they are slow to heat up and slow to cool down.